Thin-film hard disk magnetic media are widely used in read/write memory devices in computers. Increasingly, there is an effort in the thin-film medium industry to achieve higher recording density (Howard). Among the magnetic properties which are important to a high recording density are:
(1) Coercivity, defined as the magnetic field required to reduce the remanence magnetic flux to 0, i.e., the field required to erase a stored bit of information. Higher coercivity in a medium allows adjacent recoded bits to be placed more closely together without mutual cancellation. Thus, higher coercivity is associated with higher information storage density.
(2) Magnetic remanence, which determines the signal amplitude which can be read from an isolated pulse stored in the medium--the greater the remanence, the greater the signal amplitude which can be detected in a reading operation.
(3) Loop squareness, or the ratio of coercivity to saturation field. It can be appreciated that as the saturation field becomes smaller (approaches the coercivity), it takes less field strength to switch or "write to" the medium. In practical terms, this means that when a new signal is written over an old signal, the ratio of the old signal residual to the new signal is relatively small. This ratio is referred to as overwrite, a small overwrite ratio indicating good writability.
(4) Bit shift or peak shift, a phenomenon which refers to the broadening between voltage peaks, as well as reduction in peak amplitude, which occurs in the read voltage waveform, where the peak-to-peak broadening time is typically less than about 25 nsec. It is desired to achieve low bit shifting, inasmuch as bit shifting limits the resolution at which adjacent peaks can be read, and thus places an upper limit on recording density.
(5) Signal amplitude, or peak-to-peak amplitude of a single pulse, as a function of recording frequency. The recording density of the medium is related to the drop in signal amplitude at increasing recording frequency.
(6) Signal resolution, defined as the ratio of the high-frequency track average amplitude divided by the low-frequency track amplitude. The recording frequency at which 70% resolution is achieved represents one measure of information storage density on the disk.
Thin-film media or disks are commonly prepared by sputtering a thin magnetic film on a substrate, such as a textured, plated aluminum substrate. The disk is typically prepared by sputtering an underlayer, such as a chromium underlayer, onto the substrate surface, then sputtering a cobalt-based magnetic thin film over the underlayer. A protective, lubricating carbon overcoat may be applied over the thin-film layer by sputtering.
A variety of magnetic film alloys have been reported in thin-film media of the type just described (e.g., Kitada, Miura, Sanders, Shiroishi). U.S. Pat. No. 4,888,514 discloses a thin film disk having a cobalt-nickel layer sputtered over a chromium underlayer. A coercivity of 650 Oe (Oersteds), a saturation magnetization of greater than 10,000 Gauss, and a loop squareness ratio of greater than 0.9 were reported. Magnetic thin-film media with chromium underlayer with cobalt-nickel or cobalt-nickel-chromium alloy magnetic layers are also disclosed in U.S. Pat. Nos. 4,833,044, 4,816,127, and 4,735,840.